1. Field of Invention
The present invention relates to a method of fabricating a thin film transistor array substrate. More particularly, the present invention relates to a method of fabricating a thin film transistor array substrate to reduce the number of photomasks used in the processes.
2. Description of Related Art
A thin film transistor liquid crystal display (TFT-LCD) mainly comprises a thin film transistor (TFT) array substrate, a color filter array substrate and a liquid crystal layer. The TFT array substrate is constructed from a plurality of pixel structures. Each pixel structure comprises a thin film transistor and a pixel electrode corresponding to the thin film transistor. The thin film transistor comprises a gate electrode, a channel layer, a source and a drain. The thin film transistor serves as a switching element for each liquid crystal display cell.
In general, the conventional process of fabricating of a thin film transistor array substrate needs five sheets of photomasks. The first sheet of photomask is used for patterning the first metal layer to form scan lines and gates of the thin film transistor. The second sheet of photomask is used for defining channel layers and ohmic contact layers of the thin film transistors. The third sheet of photomask is used for defining second metal layers to form data lines and source/drain regions of the thin film transistors. The fourth sheet of photomask is used for patterning a passivation layer. The fifth sheet of photomask is used for patterning a transparent conductive layer to form a pixel electrode.
However, under the demands of the market, as the development of the thin film transistor liquid crystal display trends to larger scale, the process of fabricating the thin film transistor liquid crystal display confronts tremendous problems and challenges, for example, yield or through-put reduction. Hence, if the mask numbers of the process can be decreased, the steps of the process decrease. The process time can be reduced and the throughput can be increased, so that the process cost can be cut down.
U.S. Pat. No. 5,407,845 discloses a method of manufacturing thin film transistor using four sheets of photomasks. A first sheet of photomask is used for defining a first metal to form scan lines and gates of the thin film transistors. A second sheet of photomask is used for patterning active layers, i.e., channel layers, and insulating channel protective films on the active layers. Thereafter, an anisotropic etching process is conducted to remove a portion of the sidewall of the insulating channel protective films, and then the sides of the active layers are doped with ions using the insulating channel protective films as an implanting mask. A third sheet of photomask is used for patterning the second metal layer and indium tin oxide layer to form data lines, source/drain regions and pixel electrodes. A fourth sheet of photomask is used for patterning a passivation layer. The patterned passivation layers are used as a mask to remove the second metals of the pixel electrodes so as to expose the indium tin oxide layers.